Hermit Crab Shells

The shells that hermit crabs seek are made by marine gastropods that secrete 

calcium carbonate from their mantel—the organ that covers their soft bodies. 

The shell is built up in deposits until the calcium carbonate becomes a crystalline

 structure held together via thin membranes of organic material. The univalve-type

 shells that hermit crabs prefer to adopt are spiral in shape. This formation 

affords the growing gastropod within the hard shell an ever-increasing area in

 which to expand.

 Hermit crabs are scavengers and often locate these borrowed dwellings by 

smell, when the original gastropod inhabitant dies and begins to decay. Once

 a hermit crab adopts a shell, it will keep it until the shell is outgrown, carrying 

it continuously as a shield, wherever it goes. In order to carry its home, one of 

the crab's front claws is completely dedicated to clutching the shell. This claw 

bends backward and holds on to the spool of calcium carbonate at the shell's 

center. In order to move, the animal must first use this claw to lift the shell and

 heave it onto its back. In spite of such difficulties, the drive to remain housed 

is so strong in this species that a typical hermit crab would rather be torn limb 

from limb than be pulled out of its shell. The only time that the animal will 

willingly leave its shell is a) if it locates another, more suitable one, or b) if it

 is shedding its exoskeleton—a process which can only be accomplished by 

fully exiting its dwelling just long enough to wriggle out of its own exfoliated 

shell casing.

When a hermit crab that has grown too large for its current home locates a new

 one, it determines the structure's suitability via a process called fondling. During

 this activity, the hermit crab will explore the shell's surface and its internal 

volume-to-weight ratio by rolling the shell over and gently rocking it back and

 forth. Since hermit crabs actually choose the shells that they inhabit, there is a 

large body of information concerning shell selection.

Source: www.cabinetmagazine.com